1. Field
The present invention relates generally to a satellite positioning system (SPS), and more particularly, to assisting a mobile station to locate a satellite using an efficient messaging format containing extended SPS orbit correction information.
2. Background Information
A satellite positioning system (SPS) receiver normally determines its position by computing times of arrival of signals transmitted simultaneously from multiple satellites. These satellites transmit, as part of their messages, both satellite positioning data and satellite clock timing data. The satellite positions and clock timing typically are represented by almanac or ephemeris data. The ephemeris data provides an extremely accurate estimate (˜1 meter error) of satellite positions and clock bias. However, the process of searching for and acquiring satellite signals, reading the ephemeris data transmitted by the satellites, and computing the location of the receiver from this data is time consuming, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in miniaturized portable applications.
For example, Global Positioning Systems (GPS) determine position based on the measurement of the times of arrival at a GPS receiver antenna of the GPS signals broadcast from orbiting satellites. As stated, one disadvantage of such a system is the relatively long time needed to perform signal acquisition under certain conditions. Satellite signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and observed Doppler frequency shift. The process of an SPS receiver searching for, acquiring, and demodulating satellite signals is sometimes referred to as a “standalone” mode of operation, which can be contrasted with an “assisted” mode of operation.
In order to reduce the delay associated with a stand-alone mode of operation, information may be provided to aid an SPS or GPS receiver in acquiring a particular signal. Such assistance information permits a receiver to narrow the search space that must be searched in order to locate a signal, by providing bounds on the code and frequency dimensions. A system that employs a GPS receiver augmented with externally sourced GPS assistance data is commonly referred to as an “assisted global positioning system” (AGPS).
One example of an AGPS system includes a wireless mobile station (MS) (such as a cellular telephone) having, or in communication with, a GPS receiver, the MS in communication with one or more base stations (BSs), also referred to as base transmitting stations (BTSs) or node Bs, of a wireless communication network, which in turn communicate with one or more location assistance servers, sometimes referred to as Position Determination Entities (PDEs), Serving Mobile Location Centers (SMLCs), or the like, depending upon the communication air interface protocol. Another example of an AGPS system includes a MS or laptop, having, or in communication with, a GPS receiver, the MS or laptop capable of communication with a communication network, such as but not limited to, the Internet, through which the device ultimately communicates with a location assistance server.
The location assistance server derives GPS assistance information from one or more GPS reference receivers. The location assistance server also has access to a means of determining the approximate mobile station position. The location assistance server maintains a GPS database that contains reference time, satellite orbit almanac and ephemeris information, ionosphere information, and satellite working condition (“health”) information. The location assistance server also computes the assistance information customized for the approximate mobile station position.
Position location for a MS in an AGPS system can be determined at the MS (sometimes referred to as MS-based positioning mode) with assistance from a location assistance server. During MS-based positioning mode, when a GPS engine requires updated aiding data such as ephemeris data, almanac data regarding the location of satellites or base stations, timing information for the base stations and/or satellites, or seed position (such as, but not limited to that determined by advanced forward link trilateration (AFLT)), and so on, the next position fix will result in the mobile station contacting the communication network for data, thereby taxing the network and using power resources of the MS. Position location for a MS in an AGPS system can alternatively be determined at the location assistance server and transmitted back to the MS using information acquired by the MS (sometimes referred to as MS-assisted positioning mode). Satellite orbits in a GPS can be modeled as modified elliptical orbits with correction terms to account for various perturbations. The relative short-term ephemeris data provides a very accurate representation of the orbit of the satellite. For example, bit 17 in word 10 of GPS subframe 2 is a “fit interval” flag which indicates the curve fit interval used by the GPS control segment in determining the ephemeris parameters with “0” indicating a 4-hour fit and “1” indicating a “greater than 4 hours” fit. Furthermore, the extended navigation mode of the Block II/IIA GPS satellites guarantees the transmission of correct ephemeris parameters for 14 days to support short-term extended operation. During normal operation, the control segment provides daily uploads of the navigation (orbital) data to each satellite to support a positioning accuracy of 16 meters spherical error probable (SEP).
As described, a location assistance server has accurate orbital information available. Each ephemeris and clock correction model uploaded by the location assistance server usually covers a 4-hour time span with great accuracy. To cover a longer period of time, such as a 24-hour period, the location assistance server could send the device multiple 4-hour ephemeris and clock correction models for each of the N satellites in the constellation. However, it would require a large amount of octets to describe the satellite positions and clock errors for full constellation of satellites (e.g. 27 satellites). These lengthy messages would contribute to the lengthy processing time and are, therefore, unacceptable to most mobile device applications. This would also tax the communication network.
In addition to ephemeris data, satellites in a SPS also transmit almanac data that can be used to determine satellite positions and clock bias. The almanac data provides a truncated reduced-precision (coarse) set of the ephemeris parameters as well as coarse clock correction parameters. Consequently, raw satellite positions derived from the almanac data tend to be much less accurate (˜1 kilometer) than those derived from the detailed ephemeris data (˜1 meter). It should be noted that in general, the satellite orbits can be represented either by a coarse set (e.g., the almanac) or a precise set (e.g., the ephemeris) of orbital and satellite clock parameters.
A system and method is needed to provide extended orbital data to an SPS receiver to reduce the frequency of almanac and/or ephemeris downloads required, either from the satellite directly, or from a location assistance server.